In a reed switch, two lightweight metallic rods are hermetically sealed through opposite ends of an elongated hollow glass tube in such a manner that they extend substantially parallel to the longitudinal axis of the tube. A portion of each rod within the tube is flattened out, and these flattened portions (referred to as "reeds") are mutually positioned in such a way as to face each other across a narrow intervening gap (of the order of a fraction of a millimeter) and to overlap each other. Each rod behaves as a cantilever, since it is fixed at one end into the wall of the glass tube, but is free at its flattened end. A portion of each rod protrudes outward through the wall of the tube, to facilitate electrical contact with the portion located within the tube.
The rods, or at least their flattened portions, comprise a soft-magnetic material such as NiFe. Under the influence of an appropriate external magnetic field, which can penetrate the vitreous tube, the flattened portions thus become magnetised and mutually attracted, and will converge and physically contact one another if the external field is strong enough. In this way, the assembly can act as a magnetically-triggered electrical switch. Moreover, if the required magnetic field is generated by an electrical coil wrapped around the glass tube, then the reed switch can be used as an electrical relay.
The value of the external magnetic field strength at which the reeds are caused to contact each other is referred to as the AW-value of the reed switch. The term "AW" is an abbreviation for "Ampere Winding", in reference to the fact that the magnetic field strength generated along the longitudinal axis of a current-carrying coil is determined both by the magnitude of the electrical current in the coil and by the fashion in which the coil is wound (number of turns per axial unit of length, and the radius of the turns). The AW-value of the reed switch is inter alia a sensitive function of the size of the gap between the flattened portions.
Reed switches are usually manufactured in batches according to the specifications of a given customer, and a particular customer's stipulated admissible range of AW-values is generally the most crucial quality-control factor in meeting each order. Despite exercising the greatest of care and accuracy in the manufacturing process, however, the very nature of the reed switch itself results in inevitable deviations from the desired gap-value (and thus AW-value) for a given batch, since automated sealing of lightweight and easily-pliable metallic rods into the walls of a tiny glass envelope can hardly be expected to occur with an absolute guarantee of a strictly maintained mutual orientation, degree of overlap and intervening gap. As a result, a typical batch of reed switches will demonstrate a (Gaussian) distribution of gap-values around the intended gap-value.
Such a distribution of gap-values is obviously wasteful, since only a limited fraction of the obtained values will correspond to an AW-value which is acceptable for a particular customer. The manufacturer is therefore generally required to make each given batch much larger than the corresponding actual order-size, to pick out a sufficient quantity of switches within given specifications, and to sort, label and store the remaining "rejected" switches in the hope that they can be used to fill another order. Such a scenario is highly uneconomic.